Conventional manufacturing techniques for assembling airplane wing spars and ribs to a specified contour rely on fixtured "hardpoint" tooling techniques utilizing floor assembly jigs and templates to locate and temporarily fasten detailed structural parts together to locate the parts correctly relative to one another. This traditional tooling concept usually requires primary assembly tools for each subassembly produced, and two large spar assembly tools (left and right) in which the subassemblies are assembled into an assembled spar.
The tooling is intended to accurately reflect the original engineering design of the product, but using the conventional tooling concept in which the tooling sets the configuration of the final assembly, there are many steps between the original design of the product and the final manufacture of the tool. It is not unusual that the tool as finally manufactured produces missized spars or wing components that would be outside of the dimensional tolerances of the original spar or spar component design without extensive, time consuming and costly hand work to correct the tooling-induced errors. More seriously, a tool that was originally built within tolerance can become out of tolerance from the hard use it typically receives in the factory. Moreover, dimensional variations caused by temperature changes in the factory can produce a variation in the final part dimensions as produced on the tool, particularly when a large difference in the coefficient of thermal expansion exists between the tooling and the spar, as in the usual case where the tooling is made of steel and the spar components are made of aluminum. Since dimensions in airplane construction are often controlled to within 0.005", temperature induced dimensional variations can be significant.
Hand drilling of the part on the tool can produce holes that are not perfectly round or normal to the part surface when the drill is presented to the part at an angle that is slightly nonperpendicular to the part, and also when the drill is plunged into the part with a motion that is not perfectly linear. Parts can shift out of their intended position when they are fastened in non-round holes, and the nonuniform hole-to-fastener interference in a non-round hole lacks the strength and fatigue durability of round holes. The tolerance buildup on the spar subassemblies can result in significant growth from the original design dimensions, particularly when the part is located on the tool at one end of the part, forcing all of the part variation in one direction instead of centering it over the true intended position.
Spar components are typically fastened together with high interference fasteners and/or fasteners in cold worked holes. Interference fasteners, such as rivets and lock bolts, and cold working of a fastener hole, both create a pattern of stress in the metal around the hole that improves the fatigue life of the assembled joint, but a long line of such stress patterns causes dimensional growth of the assembly, primarily in the longitudinal direction, and also can cause an elongated part to warp, or "banana" along its length. Attempts to restrain the assembly to prevent such distortion are generally fruitless, so the most successful technique to date has been to attempt to predict the extent of the distortion and account for it in the original design of the parts, with the intent that the assembly will distort to a shape that is approximately what is called for in the design. However, such predictions are only approximations because of the naturally occurring variations in the installation of fasteners and the cold working of holes, so there is often a degree of unpredictability in the configuration of the final assembly. A process for washing out the effects of the distortion in the subassemblies before they are fastened into the final assembly has long been sought and would be of significant value in spar manufacturing, as well as in the manufacture of other parts of the airplane.
Wing major spar tooling is expensive to build and maintain within tolerance, and requires a long lead time to design and build. The enormous cost and long lead time to build wing spar tooling is a profound deterrent to redesigning the wing of an exist model airplane, even when new developments in aerodynamics are made, because the new design would necessitate rebuilding the wing spar tools. One existing system for automatic drilling, fastener installation and tightening is shown in U.S. Pat. No. 5,479,698 by Banks et al. entitled "Automated Spar Assembly Tool". It produces spars accurately, but is a costly system to build and maintain.
The capability of quickly designing and building spars for custom wings for airline customers having particular requirements not met by existing airplane models would give an airframe manufacturer an enormous competitive advantage. Currently, that capability does not exist because the cost of the dedicated wing and wing spar tooling and the factory floor space that such tooling would require make the cost of "designer wings" prohibitively expensive. However, if the same tooling that is used to make the standard wing spar for a particular model could be quickly and easily converted to building spars for custom wings that meet the particular requirements of a customer, and then converted back to the standard model or another custom wing design, airplanes could be offered to customers with wings optimized specifically to meet their specific requirements. The only incremental cost of the new wing would be the engineering and possibly some modest machining of headers and other low cost tooling that would be unique to that wing design.
The disadvantages of manufacturing processes using hard tooling are inherent. Although these disadvantages can be minimized by rigorous quality control techniques, they will always be present to some extent in the manufacture of large mechanical structures using hard tooling. A determinant assembly process has been developed for airplane fuselage manufacture, replacing hardpoint tooling with self-locating detail parts that determine the configuration of the assembly by their own dimensions and certain coordinating features incorporated into the design of the parts. This new process, shown in U.S. Pat. No. 5,560,102 entitled "Panel and Fuselage Assembly" by Micale and Strand, has proven to produce far more accurate assemblies with much less rework. Application of the determinant assembly process in airplane wing spar manufacture should yield a better process that eliminates or minimizes the use of hard tooling while increasing both the production capacity of the factory and increasing the quality of the product by reducing part variability while reducing the costs of production and providing flexibility in making fast design changes available to its customers. These improvements would be a great boon to airframe manufacturers and their customers, and improve the competitive position of the manufacturer in the marketplace. The present invention is a significant step toward such a process.